Low energy ultrahigh momentum resolution angle resolved photoemission spectroscopy study on four-layer self-doped high $T_c$ superconductor ${\mathrm{Ba}}_2{\mathrm{Ca}}_3{\mathrm{Cu}}_4{\mathrm{O}}_8{\mathrm{F}}_2$ (F0234) revealed fine structure in the band dispersion, identifying the unconventional association of hole and electron doping with the inner and outer ${\mathrm{CuO}}_2$ layers, respectively. For the states originating from two inequivalent ${\mathrm{CuO}}_2$ layers, different energy scales are observed in dispersion kinks associated with the collective mode coupling, with the larger energy scale found in the electron ($n$-) doped state which also has stronger coupling strength. Given the earlier finding that the superconducting gap is substantially larger along the $n$-type Fermi surface, our observations connect the mode coupling energy and strength with magnitude of the pairing gap.

• Received 22 January 2009

DOI:https://doi.org/10.1103/PhysRevLett.103.036403